The present invention relates to an electrochemical sensor element, in particular for determining the oxygen level in gas mixtures.
Sensor elements are known. They are designed as planar sensor elements, which have, on a solid electrolyte designed as a support, a first electrode exposed to the measured gas and a second gas exposed to a reference gas. Furthermore, an electrical resistance heater is embedded in the support. A reference gas, which is in most cases made up of atmospheric air, is supplied to the reference electrode via a reference gas channel integrated in the support. At the same time, the reference channel forms a gas chamber having a bottom surface matching the reference electrode in the reference electrode area, so that sufficient oxygen may reach the reference electrode.
It is known from European Patent No. 125069 that the width of the reference gas channel can be adapted to match the width of the electrode over its entire length for this purpose, or two reference gas channels, with one electrode arranged in each, can run in a layer plane parallel to one another, with the two electrodes connected together, forming the reference electrode. The disadvantage of a wide reference gas channel or a reference gas channel made up of two adjacent parts is that one part of the heating coil of the resistance heater element is always in the area of the perpendicular projection of the reference gas channel. This results in overheating of the solid electrolyte in the area of the reference gas channel. In addition, a wide reference gas channel provides poor heat transfer between the resistance heating element and the electrodes.
The method described in German Patent Application No. 19609323 in which the reference gas channel is branched in the area of the heating device, offers a possible remedy. However, in this case the reference electrodes must also be branched.
The sensor element according to the present invention has the advantage that it allows improved heat transfer between the electrodes and the resistance heating element, resulting in uniform heat distribution. The porous layer also helps relieve mechanical stresses that occur at the edges where the reference gas channel and the adjacent solid electrolyte film meet, and which may result in stress cracks in the ceramic support. In bridging a wide reference gas channel, the solid electrolyte film is bent, which results in additional mechanical stresses. Using the narrow reference gas channel, excessive bending of the adjacent solid electrolyte film is avoided. Furthermore, due to the large-surface contact of the reference electrode with the adjacent porous layer, better adhesion of the latter is achieved, since the reference electrode remains pressed between the adjacent films during lamination. This is also true for the lead to the reference electrode, with its resistance also being thereby reduced.
It should be emphasized that the reference gas channel may have a slightly widened handgrip shape in the area of the reference electrode. This allows oxygen exchange to be improved, in particular in the case of low pore volumes. The effect of the reference atmosphere can be intensified by adding an oxygen-storing material, for example, CeO2, to the porous layer. This can be achieved by impregnating the porous layer or the porous electrode.